1. Field of the Invention
The present invention relates to a thin-film resistor, and more particularly, to a thin-film resistor employed in a semiconductor wafer and its method of formation.
2. Description of the Prior Art
There are many kinds of resistor components in ICs of a semiconductor wafer such as the gate conducting layer of the semiconductor wafer, impurity doped layer as a resistance component, or the thin-film resistor. The main problem with the gate conducting layer and the impurity doped layer is that the resistance of both are too low. Therefore, these components, if used, must be made large to increase their resistance to sufficient levels. It is clear that the gate conducting layer and the impurity doped layer are not suitable for use in semiconductor processing with small line-widths. Also, since the gate conducting layer and the impurity doped layer comprise silicon conducting material, the conductivity of the resistance component easily varies with temperature changes making the resistance of these resistance components very unstable. If a layer of resistance component with low conductivity and stable resistance is required for an IC, the thin-film resistor is essential.
Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 are schematic diagrams of the method of forming a thin-film resistor 18 according to the prior art. A thin-film resistor 18 of the prior art is formed on the surface of the dielectric layer 10 of a semiconductor wafer 11. First, a resistance layer 12 and a protective layer 14 are sequentially formed within a predetermined area on the surface of the dielectric layer 10. Next, a conducting layer 16 made of alloy of aluminum is formed on the surface of the dielectric layer 10 and the protective layer 14, as shown in FIG. 1. Then, a wet etching process is performed to remove all of the conducting layer 16 and the protective layer 14 on the resistance layer 12 except for at the two ends of the resistance layer 12. This remaining portion is used as the electrical connecting wires of the two ends of the resistance layer 12. FIG. 2 illustrates the completed thin-film resistor 18.
The wet-etching process is an isotropic process, that is to say the side depth of etching is approximately equal to the vertical depth of etching. Since the thin-film resistor 18 patterns the conducting layer 16 by wet-etching, it is essential that the resistance layer 12 and the protective layer 14 have large surface areas so that the most of the conducting layer 16 and the protective layer 14 on the surface of the resistance layer 12 can be removed. At the same time, the conducting layer 16 and the protective layer 14 at the two ends of the resistance layer 12 are maintained. So the prior art method of forming the thin-film resistor 18 can only be used in processes with line-width of 3 .mu.m and cannot be used in processes with smaller line widths.